Standard optical microscopy is not capable of obtaining a transverse resolution with a definition better than approximately half a wavelength of light due to the diffraction limit, also termed the Rayleigh or Abbe limit. The diffraction limited spatial resolution is xcex3/(2 NA) where xcex3 is the wavelength of collected light in free space. The Numerical Aperture is defined as NA=n sinxcex8a where n is the refractive index of the material and xcex8a is the collection angle, the half-angle of the optical collection area. In order to improve resolution of diffraction limited microscopy the NA must be increased. The highest NA values for standard microscope objectives in air ambient are less than 1, with typical best values around 0.6.
One method to increase the NA is to increase the index n of the material where the collection focus is formed. Insertion of a high index fluid, such as oil, between the microscope objective lens and the sample allows for higher NA, with typical best values around 1.3. Similarly, a microscope design utilizing a high index hemispherical lens, called a Solid Immersion Lens (SIL), closely spaced to the sample can provide a resolution improvement of 1/n. The SIL microscope relies on evanescent coupling between the light focussed in the high index SIL AND THE SAMPLE. Previous patents on SIL microscopy describe arrangements where the light is focussed at the geometrical center of the spherical surface of the SIL.
Subsurface imaging of planar samples is normally accomplished by standard microscopy. The NA remains the same when imaging below the surface of higher index samples, because the increase in index is exactly counterbalanced by the reduction of sinxcex8a from refraction at the planar boundary. Standard subsurface imaging also imparts spherical aberration to the collected light from refraction at the same planar boundary. The amount of spherical aberration increases monotonically with increasing NA. Subsurface imaging has been conducted through Silicon substrates at wavelengths of 1.0 xcexcm and longer, with best values of transverse resolution around 1.0 xcexcm.
The use of SIL microscopy has been suggested for subsurface imaging wherein the light phase fronts are geometrically matched to the SIL surface. However, the method described is limited to an arrangement where a hemispherical lens collects light from a focus at the geometrical center of the spherical surface of the lens. In this case, the resolution improvement is limited to 1/n, and the spherical aberration free area is limited to a point. An image can be formed by scanning the sample and SIL where the scan precision is relaxed by a factor of n. An image can also be formed by scanning the sample and holding the SIL stationary. The characteristics of the invention described below are an improvement over those of standard and SIL microscopy for many sub-surface applications.
The present invention provides a substrate surface placed lens for viewing or imaging to or from a zone of focus within the substrate and providing an increase in the numerical aperture of the optical system over what it would be without the lens. The enhanced numerical aperture translates into an improvement in resolution in collecting or illuminating. The focus at a specific zone within the substrate is made aberration free, providing a broad lateral extent to the field of view. Substrate and lens material are close if not identical in index of refraction, n.
The invention finds application in viewing semiconductor devices and circuits, bio/chem specimens from the underside of an attachment surface, layered semiconductor and dielectrics such as boundaries of Silicon-on-Insulator substrates, and read/write functions of buried optical media.